1. Field of the Invention
The present invention relates to a single mode fiber optical transmitter system capable of suppressing stimulated Brillouin scattering. In particular, the invention relates to a high power transmitter that broadens the spectral width of its laser source, a modulator or both by noise insertion or by multiple beat frequency insertion. Each of the multiple beat frequencies has a corresponding reduced amplitude but has the same laser line width, thus increasing power that can be injected into the fiber before the onset of stimulated Brillouin scattering.
2. Description of Related Art
High power laser transmitters are required to transmit high quality AM-VSB video channels from point to point over long single mode optical fiber links. For example, long line transmission of information channels is required from headend to headend or from headend to primary hubs in cable television networks where information may be broadcast to several nodes. In FIG. 7, known optical transmission system 10 includes optical transmitter 12, fiber link 14 and optical receiver 16. Optical transmitter 12 includes modulation section 20 and amplifier 18. Modulation section 20 includes laser source 22 and signal modulator 26.
Where high optical powers are launched into long single mode fiber lengths non-linear effects such as stimulated Brillouin scattering (SBS) of the fiber degrades signal quality. Both received optical signal power is reduced, and scattering noise is induced at the receiver.
The degree of signal quality degradation generally depends on the laser source line width, the launched optical power and the system fiber length. This degradation is minimal below a threshold power density level which is about 6.2 dBm for externally modulated optical transmitters. This SBS gain threshold is increased proportionally when the source line width increases.
Known semiconductor DFB and solid state laser sources have narrow line widths. System signal quality (i.e., system carrier-to-noise ratio measured at the optical receiver) is increased when techniques are used to broaden the laser source line width in long distance optical fiber transmission systems. As the laser source line width is increased, the optical power density of the information carrying optical signal is reduced, and the SBS gain threshold is increased.
In FIG. 8, known modulation section 20 (i.e., from system 10 of FIG. 7), includes laser source 22 to generate the optical signal. The optical signal is coupled to signal modulator 26 where it is modulated according to information signal S.sub.IN. Modulation section 20 also includes continuous wave (CW) source 28 coupled to laser source 22. Laser source 22 is modulated according to a signal from CW source 28. This broadens the effective spectral width from laser source 22 by imparting a periodic frequency modulation to the optical frequency of the laser source.
In U.S. Pat. No. 4,477,368 to Eskildsen et al. and U.S. Pat. No. 5,329,396 to Fishman et al., both incorporated herein by reference, there is described a dither signal from a CW source to directly modulate the laser source where the dither signal is either a sinusoidal or square waveform. In U.S. Pat. No. 5,359,450 to Ramachandran et al., incorporated herein by reference, there is described a distributed feedback laser source modulated by a CW signal having a frequency of about 1 GHz. In U.S. Pat. No. 5,453,868 to Blauvelt et al., incorporated herein by reference, there is described a laser source modulated by a chirp generating signal (e.g., sawtooth waveform).
In FIG. 9, known modulation section 20 includes laser source 22 optically coupled to phase modulator 24 optically coupled in turn to signal modulator 26. Modulation section 20 also includes broad band noise source 30 coupled to phase modulator 24. Phase modulator 24 is modulated according to a signal from broad band noise source 30. This broadens the effective spectral line width of the optical signal coupled from phase modulator 24 to signal modulator 26 by imparting a random frequency modulation to the optical frequency input to signal modulator 26.
In U.S. Pat. No. 5,166,821 to Huber, incorporated herein by reference, there is described a broad band electrical noise source coupled to an optical modulator.
In FIG. 10, known modulation section 20 includes laser source 22 optically coupled to phase modulator 24 optically coupled in turn to signal modulator 26. Modulation section 20 also includes CW source 32 coupled to phase modulator 24. Phase modulator 24 is modulated according to a signal from CW source 32. This broadens the effective spectral line width of the optical signal coupled from phase modulator 24 to signal modulator 26 by imparting a periodic frequency modulation to the optical frequency input to signal modulator 26.